Method for improving the properties of precipitation hardenable castings



Patented May 24, 1949 METHOD FOR IMPROVING THE PROPER- TlES OF BltliQIPI'rATIoN HARDENABLE CAST NQ$ William A. Reich, Sclcrenectady N. 12, assignor to General Electric Com New York p rty, a corporation of No Drawing. Application April 22, 1944, Serial No. 532,360

Claims. (01. 14 -115) The present invention relates to aged or precipitation ha denahl casting and more i ularly to a process for improving the properties of such castings. In many applications of precipitation hardenable castings it is desirable to have a reasonable ductility and strength in order that the casting may function in a satisfactory manner. This is particularly true in castings in hite st es ne n raiiens r e u in th neighborhood of micro-structural discontinuities Su h t sla in l s ens fine er t rough s rfaces, etc. In service suchdiscontinuities or stress he e. may caus th la ti mit o the ast n to be exceedednq er .cqe i i ne he 31 19- tile material will flow or be permanently deformed in the neighborhood of the discontinuity and will cause the stress concentration to be alleviated. 0n the other hand, a brittle material may sufier incipient failure in the location of the discontinuity and this may lead to a failure of the casting.

In most high strength cast aluminum alloys the ductility is low. The high strength properties in such alloys usually are obtained by means of a process comprising a high temperature solution treatment followed successively by a quench and a relatively low temperature aging or precipitation treatment. In the quenched condition the ductility of the alloy is usually quite high but the strength is low. The aging or precipitation treatment effects an increase in strength but lowers the ductility of the casting.

It is one of the objects of the present invention to provide a process whereby micro-stress concentrations in castings may be substantially eliminated. It is a further object of the invention to provide a process whereby a limited amount of stress may be applied to a precipitation hardenable casting while it is in the quenched ductile condition to thereby eliminate structural discontinuities in the casting. Other objects will appear hereinafter.

In carrying out my invention the precipitation hardenable casting is given the usual high temperature solution treatment followed by a quench in hot or cold water. After the quench any machining of the casting may be completed as far as possible. Thereafter the casting is prestressed in a manner simulating service conditions as closely as possible but within the physical limits of the quenched casting. The prestresses developed under such conditions are suflicient to cause the elastic limit to be exceeded in the neighborhood of any stress concentrations so that they are relieved or substantially eliminated by localized plastic flow. The casting is thereafter subjected to the usual aging or precipitation treatment to increase the strength of the casting. The casting should then be ready for service Without the low ductility of the casting in the aged condition being the cause of serious deficiencies.

The prestressing applied for example to a super.- charger impeller may be determined from the bursting speed of an impeller in the solution treated'and quenched condition. The stress" in.- troduced into an impeller by rotation is proportional to the square of the speed of rotation, The typical properties of precipitationhardenable a1,- loysinthe as quenched condition are known. Having determined the bursting speed (5 in the solution treated and quenched condition, the prestressing speed (Sp) may be determined as follows:

G SB s,

S =Prestressing speed SB=Bursting speed Gp=A stress 10% above the normal elastic limit GB=Tensile strength All values are for solution treated and quenched condition.

A cast precipitation hardened aluminum alloy supercharger impeller attained a bursting speed of 27,600 R. P. M. Supercharger impellers made from the identical alloy were given a solution and quench treatment and then prestressed by operating for about one and one-half minutes at 22,000 R. P. M. After this prestressing the castings were given a normal, artificial aging treatment. When subsequently tested they attained bursting speeds of 31,800 and 31,500 R. P. M. Since stress is proportional to the square of the speed of rotation these results indicate that the prestressed impellers withstood stresses about 25% higher than the impellers which were not prestressed.

The prestressing operation employed in the present process should be distinguished from the cold Working step sometimes employed to improve the strength of precipitation hardenable castings. Cold working under such circumstances comprises permanent deformation of an entire casting. In the present prestressing operation however the elastic limit of the material is only slightly exceeded whereby micro-stress concentrations in the casting are eliminated without appreciably affecting the shape or dimensions of the casting.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The method for improving a precipitation hardenable alloy casting which comprises subjecting said casting in the ductile solution heat treated and quenched condition to a prestress slightly beyond its elastic limit whereby microstress concentrations in said casting are eliminated and thereafter aging said casting, said prestress simulating the stress encountered by said casting under service conditions.

2. The method for improving the properties of a rotatable precipitation hardenable alloy casting which comprises rotating the casting in the quenched and ductile solution heat treated condition at a speed about 25% lower than its bursting speed and then aging the casting.

3. The method for improving the properties of a rotatable precipitation hardenable alloy casting which comprises rotating the casting in the quenched and ductile solution heat treated condition for a few minutes at a speed lower than its bursting speed but high enough to remove micro-stress concentrations in said casting and thereafter aging said casting.

4. The method for improving a rotatable precipitation hardenable alloy casting which comprises rotating the casting in the quenched and ductile solution heat treated condition at a speed of rotation sufficient to stress said casting slightly beyond its elastic limit whereby micro-stress concentrations in said casting are eliminated without causing appreciable change in the shape or dimensions of said casting and thereafter aging said casting.

S =Prestressing speed SB=Bursting speed Gp=A stress 10% above the normal elastic limit GB=Tensile strength, and thereafter aging said casting.

WILLIAM A. REICH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,891,234 Langenberg Dec. 1932 1,899,631 Norton Feb. 1933 2,116,923 Bolton May 10, 1938 OTHER REFERENCES The Aluminum Industry, vol. 2, by Edwards et al., page 425, 1930. p

The Metal Industry, pages 335-340, November 1, 1935, and pages 464-468, November 8, 1935.

Age Hardening of Metals, The American Society for Metals, page 320, 1940. 

